This invention relates to semiconductor integrated circuits and particularly to the type having relatively precisely valued capacitors formed integrally therewith. These capacitors are of the type composed of a sandwich of polycrystalline silicon, thermal oxide, and another layer of polycrystalline silicon. Polycrystalline silicon hereinafter will be referred to as polysilicon. Capacitors of this type, termed polyoxide capacitors, are important parts of certain semiconductor integrated circuits such as filter/codecs. These capacitors are chiefly used in devices fabricated using MOS technology and in the case of many applications, such as designs which include switched capacitor filters, occupy large portions of the semiconductor chip.
Semiconductor chip area is a valuable commodity, and there is a continual effort to achieve more compact design of both devices and interconnections. Capacitance is a direct function of area and an inverse function of the thickness of the dielectric. If the thickness of the dielectric oxide layer of polyoxide capacitors can be reduced, there can be a reduction in the area of the capacitor for a given capacitance.
However, the realization of higher capacitance devices from the use of thinner dielectric oxide layers is inhibited by the fact that dielectric integrity for films of about 600 Angstroms or less cannot be assured above an electric field strength of about 3.5 MV/cm. The structure of the oxide constituting the dielectric of the polyoxide capacitor appears to be a function of the surface morphology of the polysilicon layer upon which the thermal oxide is grown. The figure given for breakdown is for structures in which the polysilicon has been made conductive by the customary practice of diffusing an impurity such as phosphorus into the polysilicon layer at a relatively high temperature after the polysilicon layer has been deposited. The polysilicon layer usually is deposited above or near the transition temperature from amorphous to crystalline structure, and thus results in a columnar crystalline structure with a relatively rough surface. This appears to considerably affect the character of the polysilicon layer surface.
Accordingly, it is a purpose of this invention to provide a technology for the fabrication of polyoxide capacitors which will result in higher electrical breakdown fields. An ancillary object is the fabrication of polyoxide capacitors of smaller area for a given value of capacitance.